JP2017045537A - Manufacturing method of negative electrode for lithium ion secondary battery and negative electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium ion secondary battery with improved Li precipitation resistance.SOLUTION: A manufacturing method of a negative electrode for a lithium ion secondary battery includes: a first step (S101) of preparing a negative electrode mixture containing at least a negative electrode active material and lithium tungstate; and a second step (S102) of producing the negative electrode for a lithium ion secondary battery by using the negative electrode mixture. In the first step, the negative electrode mixture is prepared so that the negative electrode mixture contains lithium tungstate of greater than 0 μmol/g and less than or equal to 201 μmol/g.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a negative electrode for a lithium ion secondary battery and a negative electrode for a lithium ion secondary battery.

  JP-A-2015-022913 (Patent Document 1) discloses a negative electrode active material capable of reversibly inserting and desorbing lithium (Li) ions.

Japanese Patent Laying-Open No. 2015-022913 JP 2004-063123 A

Graphite (graphite) is known as a negative electrode active material for lithium ion secondary batteries. Graphite has a crystal structure in which a plurality of sheets (called “graphene”) in which carbon atoms are bonded in a honeycomb shape by sp ² bonds are stacked. In graphite, Li ions can be reversibly inserted and desorbed in the gaps between graphene.

  A lithium ion secondary battery is a typical rocking chair type battery. That is, in the lithium ion secondary battery, the concentration of Li ions (charge carriers) in the electrolytic solution is substantially constant regardless of the state of charge of the battery. For example, during charging, Li ions in the electrolytic solution are inserted into the negative electrode active material by supplying current. At the same time, the same amount of Li ions as the amount of Li ions inserted into the negative electrode active material is released from the positive electrode active material into the electrolytic solution, and the Li ion concentration in the electrolytic solution is kept substantially constant.

  However, in practice, the Li ion concentration in the electrolytic solution is not always homogeneous and may include partial bias. For example, in a vehicle-mounted lithium ion secondary battery, pulse wave-shaped large current charging may be performed in a short time. At this time, depending on the magnitude of the charging current, the movement of Li ions in the electrolytic solution may not catch up with the charging current, and Li ions may be insufficient around the negative electrode active material. If there is a shortage of Li ions to be inserted into the negative electrode active material around the negative electrode active material, large polarization occurs. As a result, the negative electrode potential may be lowered and reach the deposition potential of Li metal.

  Due to these concerns, in many cases, the upper limit of the charging current is set low in the lithium ion secondary battery. For this reason, for example, in a hybrid vehicle or the like, there is a case where the recovery of the regenerative power is lost. This is because charging current (input) exceeding the upper limit is limited.

  Therefore, an object of the present invention is to provide a negative electrode for a lithium ion secondary battery in which Li precipitation hardly occurs, that is, Li precipitation resistance is improved.

  [1] A method for producing a negative electrode for a lithium ion secondary battery includes a first step of preparing a negative electrode mixture containing at least a negative electrode active material and lithium tungstate, and a lithium ion secondary battery using the negative electrode mixture A second step of manufacturing a negative electrode. In the manufacturing method, in the first step, the negative electrode mixture is prepared so that the negative electrode mixture contains lithium tungstate exceeding 0 μmol / g and not more than 201 μmol / g.

In the production method [1], lithium tungstate (typically “Li ₂ WO ₄ ”) is added to the negative electrode mixture. In the negative electrode mixture thus prepared, lithium tungstate is present around the negative electrode active material. For this reason, the amount of Li ions present around the negative electrode active material is increased, and it is considered that the decrease in the negative electrode potential due to the lack of Li ions is suppressed during large current charging. Furthermore, since the tungstate ion from which Li ions are released has an unshared electron pair, it can be expected to increase the nucleophilicity of the negative electrode active material, that is, the reactivity between the negative electrode active material and Li ions. Thereby, it is considered that the insertion reaction of Li ions into the negative electrode active material proceeds smoothly. As a result of these synergies, it is considered that the lithium deposition resistance is improved in the negative electrode for a lithium ion secondary battery produced by the production method of [1] above.

  However, the amount of lithium tungstate added to the negative electrode mixture is 201 μmol / g or less. This is because if it exceeds 201 μmol / g, the Li precipitation resistance may be lowered.

  [2] In the first step, the negative electrode mixture is preferably prepared so that the negative electrode mixture contains 20 μmol / g or more of lithium tungstate. In such a range, improvement in Li precipitation resistance can be expected.

  [3] In the first step, it is preferable to prepare the negative electrode mixture such that the negative electrode mixture further contains a film forming agent that forms a film on the surface of the negative electrode active material by an electrochemical reduction reaction. The film forming agent forms a film on the surface of the negative electrode active material, for example, at the first charge. It is considered that the lithium tungstate can be fixed to the surface of the negative electrode active material by the coating. In addition, the coating can be expected to have an effect of suppressing elution of lithium tungstate during, for example, high temperature storage. Thereby, the effect which suppresses the fall of Li precipitation tolerance at the time of high temperature preservation | save etc. can be expected.

  [4] The film forming agent is preferably vinylene carbonate. The coating derived from vinylene carbonate is considered to have a large effect of suppressing elution of lithium tungstate.

  [5] In the first step, the value obtained by dividing the content of vinylene carbonate in the negative electrode mixture by the content of lithium tungstate in the negative electrode mixture is 0.85 or more and 1.2 or less. It is preferable to prepare a negative electrode mixture. This is because, within such a range, the effect of suppressing a decrease in Li precipitation resistance tends to be large.

  [6] The film forming agent may be lithium bis (oxalate) borate. Even a coating derived from lithium bis (oxalate) borate can be expected to suppress the elution of lithium tungstate.

  [7] A negative electrode for a lithium ion secondary battery includes a negative electrode mixture containing at least a negative electrode active material and lithium tungstate. The negative electrode mixture contains lithium tungstate exceeding 0 μmol / g and not more than 201 μmol / g. As described above, the negative electrode mixture to which lithium tungstate is added can be expected to improve Li precipitation resistance.

  [8] The negative electrode mixture preferably contains 20 μmol / g or more of lithium tungstate. In such a range, improvement in Li precipitation resistance can be expected.

  According to the above, a negative electrode for a lithium ion secondary battery having improved Li precipitation resistance is provided.

It is a flowchart which shows the outline of the manufacturing method of the negative electrode for lithium ion secondary batteries which concerns on embodiment of this invention. It is a flowchart which shows the outline of the manufacturing method of a lithium ion secondary battery. It is the schematic which shows an example of a structure of the negative electrode for lithium ion secondary batteries which concerns on embodiment of this invention. It is the schematic which shows an example of a structure of a positive electrode. It is the schematic which shows an example of a structure of an electrode group. It is a schematic sectional drawing which shows an example of a structure of a lithium ion secondary battery. It is a graph which shows the relationship between content of lithium tungstate in a negative electrode compound material, and a limiting current value. It is a graph which shows the relationship between the value which remove | divided content of vinylene carbonate by content of lithium tungstate, and the limiting current value after high temperature storage.

  Hereinafter, an example of an embodiment of the present invention (hereinafter may be referred to as “this embodiment”) will be described. However, this embodiment is not limited to the following description. In the following description, a negative electrode for a lithium ion secondary battery may be simply referred to as a “negative electrode”, and a lithium ion secondary battery may be simply referred to as a “battery”.

<Method for producing negative electrode for lithium ion secondary battery>
FIG. 1 is a flowchart showing an outline of a manufacturing method according to this embodiment. As shown in FIG. 1, the manufacturing method includes a first step (S101) and a second step (S102). Hereinafter, each step will be described.

<< First Step (S101) >>
In the first step, a negative electrode mixture containing at least a negative electrode active material and lithium tungstate is prepared. First, various materials that should constitute the negative electrode composite are prepared.

(Negative electrode active material)
The negative electrode active material is typically graphite. In addition to graphite, the negative electrode active material may be graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), or the like. That is, the negative electrode active material may be at least one selected from the group consisting of graphite, graphitizable carbon, and non-graphitizable carbon. Here, the easily graphitizable carbon indicates a carbon material that easily forms a regular crystal structure similar to graphite by a heat treatment at 2500 ° C. or higher in an inert atmosphere. Further, non-graphitizable carbon is a carbon material that hardly forms a regular crystal structure similar to graphite even under the same conditions.

  The negative electrode active material may be, for example, particles obtained by spheroidizing natural scaly graphite. The spheroidization treatment refers to, for example, a treatment in which particles are folded by applying stress to the scaly graphite particles in parallel with the graphene plane so that the outer shape of the particles approaches a sphere. The spheronization process may involve partial grinding. Specifically, for example, a method of giving an impact to the scaly graphite particles in a high-speed air flow can be considered. The spheroidized graphite particles may be further coated with amorphous carbon. That is, the negative electrode active material may be composite particles including spheroidized graphite particles and amorphous carbon attached to the surfaces of the spheroidized graphite particles.

The average particle diameter of the negative electrode active material is preferably 5 μm or more and 15 μm or less. The average particle size in the present specification indicates a particle size (also referred to as “d50”, “median diameter”, etc.) at a cumulative value of 50% in a volume-based particle size distribution measured by a laser diffraction scattering method. Moreover, the BET specific surface area of the negative electrode active material is preferably 2.9 m ² / g or more and 5.1 m ² / g or less. In such a range, the balance between the large current charging characteristic and the storage characteristic is good. The BET specific surface area indicates a value measured by a BET method using nitrogen gas.

(Lithium tungstate)
The lithium tungstate is typically Li ₂ WO ₄ . However, the form of lithium tungstate is not necessarily limited to Li ₂ WO ₄ . Lithium tungstate can take the form of Li ₄ WO ₅ , Li ₆ W ₂ O _9, etc., for example. Moreover, the lithium tungstate may be a hydrate.

  The average particle size of lithium tungstate is preferably smaller than the average particle size of the negative electrode active material. Thereby, when mixing a negative electrode active material and lithium tungstate, it is thought that it becomes easy to arrange | position lithium tungstate on the surface of a negative electrode active material. By arranging lithium tungstate on the surface of the negative electrode active material, it is considered that the effect expected in the present embodiment is increased.

  The average particle size of lithium tungstate is preferably 0.1 μm or more and 5 μm or less. By using lithium tungstate having an average particle diameter of 0.1 μm or more, it becomes difficult to include fine powder having low Li ion conductivity, and therefore, it is possible to expect an improvement in large current charging characteristics. Moreover, it is considered that by using lithium tungstate having an average particle diameter of 5 μm or less, lithium tungstate is less likely to become a steric hindrance in the negative electrode mixture, and a good conductive path is likely to be formed.

(Other ingredients)
In addition to the above-described negative electrode active material and lithium tungstate, examples of components constituting the negative electrode mixture include a thickener and a binder.

  The thickener may be, for example, carboxymethylcellulose (CMC), sodium salt of carboxymethylcellulose (CMC-Na), polyacrylic acid (PAA), sodium polyacrylate (PAA-Na), polyvinyl alcohol (PVA), and the like. The binder may be, for example, styrene butadiene rubber (SBR), acrylonitrile butadiene rubber (NBR), polytetrafluoroethylene (PTFE), or the like.

(Film forming agent)
Furthermore, in this embodiment, it is preferable to contain in the negative electrode mixture a film forming agent that forms a film on the surface of the negative electrode active material by an electrochemical reduction reaction. The electrochemical reduction reaction can occur, for example, when the battery is first charged. Conventionally, such a film-forming agent is added to the electrolytic solution. It is considered that the effect of fixing lithium tungstate to the negative electrode active material is increased by including the film forming agent directly in the negative electrode mixture instead of the electrolyte solution. In addition, it is considered that the elution of lithium tungstate can be suppressed by the coating, for example, during high temperature storage.

The film forming agent is preferably a compound having a reduction potential in the range of, for example, about 0.5 to 1.5 V (vs. Li ⁺ / Li). Examples of the film forming agent include lithium bis (oxalate) borate [LiB (C ₂ O ₄ ) ₂ ; abbreviation “LiBOB”], lithium difluorooxalate borate [LiBF ₂ (C ₂ O ₄ )], lithium difluorobis (oxalate) ) Phosphate [LiPF ₂ (C ₂ O ₄ ) ₂ ], lithium difluorophosphate (LiPO ₂ F ₂ ), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), ethylene sulfite (ES ), Propane sultone (PS) and the like. These film forming agents may be used alone or in combination of two or more. That is, the film forming agent is at least one selected from the group consisting of LiBOB, LiBF ₂ (C ₂ O ₄ ), LiPF ₂ (C ₂ O ₄ ) ₂ , LiPO ₂ F ₂ , VC, VEC, FEC, ES, and PS. It may be a seed.

  According to the study of the present inventor, among the film forming agents described above, a particularly great effect can be expected in VC and LiBOB. That is, the film forming agent is preferably at least one of VC and LiBOB.

(Method for preparing negative electrode composite)
In the present embodiment, the method for preparing the negative electrode mixture is not particularly limited. For example, the negative electrode mixture may be prepared by dry mixing, or the negative electrode mixture may be prepared by wet mixing. The negative electrode mixture may be prepared to be a paint (referred to as “slurry”, “paste”, etc.) by, for example, being dispersed in a solvent. Or you may prepare so that a negative electrode compound material may become a granulated body (aggregate of granules) by granulating a negative electrode compound material. Below, an example of a suitable procedure is shown.

  First, each component that should constitute the negative electrode mixture is weighed. The composition of each component in the negative electrode mixture is, for example, as follows.

Thickener: about 0.1 to 2% by mass Binder: about 0.1 to 2% by mass Lithium tungstate: more than 0 μmol / g and not more than 201 μmol / g Negative electrode active material: remainder other than the above components (typical 90 to 99% by mass).

  As described above, in the present embodiment, the negative electrode mixture is prepared so that the negative electrode mixture contains lithium tungstate exceeding 0 μmol / g and not more than 201 μmol / g. In such a range, improvement in Li precipitation resistance can be expected.

  In the present embodiment, it is preferable to prepare the negative electrode mixture such that the negative electrode mixture contains 20 μmol / g or more and 201 μmol / g or less of lithium tungstate. As will be described later, according to the study by the present inventors, it has been confirmed that the improvement range of Li precipitation resistance is large in the range of 20 μmol / g or more and 201 μmol / g or less.

  According to the study, the range in which the improvement width of Li precipitation resistance is larger is considered to be a range of 20 μmol / g to 135 μmol / g. That is, in the present embodiment, it is more preferable to prepare the negative electrode mixture so that the negative electrode mixture contains 20 μmol / g or more and 135 μmol / g or less of lithium tungstate.

  According to the examination, the range in which the improvement width of Li precipitation resistance is even greater is considered to be a range of 20 μmol / g or more and 80 μmol / g or less. That is, in the present embodiment, it is even more preferable to prepare the negative electrode mixture such that the negative electrode mixture contains 20 μmol / g or more and 80 μmol / g or less of lithium tungstate.

  Furthermore, according to the same study, it is considered that the range in which the improvement width of Li precipitation resistance is the largest is a range of 32 μmol / g or more and 64 μmol / g or less. That is, in this embodiment, it is most preferable to prepare the negative electrode mixture so that the negative electrode mixture contains lithium tungstate in a range of 32 μmol / g to 64 μmol / g.

  In the present embodiment, the film forming agent is expected to have an effect of fixing lithium tungstate to the surface of the negative electrode active material. Therefore, it is considered that the content of the film forming agent in the negative electrode mixture is desirably determined from the relationship with the content of lithium tungstate in the negative electrode mixture. According to the study of the present inventor, when the film forming agent is VC, a value obtained by dividing the content of VC in the negative electrode mixture by the content of lithium tungstate in the negative electrode mixture is 0.85 or more. The negative electrode mixture is preferably prepared so as to be 2 or less. For example, when the content of lithium tungstate is 20 μmol / g or more and 201 μmol / g or less, the content of VC is preferably 17 μmol / g or more and 241.2 μmol / g or less. This is because in such a range, the elution suppression effect of lithium tungstate tends to be large. The value obtained by dividing the content of VC in the negative electrode mixture by the content of lithium tungstate in the negative electrode mixture is more preferably 0.90 or more and 1.2 or less.

  For the preparation of the negative electrode mixture, for example, a general mixing device such as a planetary mixer, a kneader, or a homogenizer, a stirring device, or the like can be used. From the viewpoint of more uniformly dispersing lithium tungstate in the negative electrode mixture, it is desirable to prepare the negative electrode mixture through a kneading step and a dilution dispersion step as described below.

  In the kneading step, it is preferable to mix the negative electrode active material, the thickener and lithium tungstate by feeding them into the mixing container of the mixing apparatus. When a film forming agent is used, it is preferable to add the film forming agent in the kneading step. Such an aspect may increase the effect expected of the film forming agent.

  In the kneading step, the mixture is kneaded while applying a strong shearing force in a state where the solid content ratio of the mixture is high (that is, a state where the viscosity is high). Here, solid content ratio has shown the mass ratio which components other than a solvent occupy in a mixture. The solid content ratio in the kneading step may be about 60 to 80% by mass, for example. The mixing conditions (mixing time, rotation speed of the stirring blade, etc.) in the kneading step can be appropriately changed according to the powder physical properties of the mixture.

  In the dilution dispersion step, a solvent is added to the mixture, and the negative electrode mixture is dispersed in the solvent in a state where the solid content ratio is lowered (that is, in a state where the viscosity is low). The solid content ratio in the dilution and dispersion step is, for example, about 40 to 60% by mass. It is desirable to add the binder in the dilution / dispersion step. Such an aspect may improve the large current charging characteristics. The mixing conditions in the dilution / dispersion step can be appropriately changed according to the viscosity of the mixture. For example, in the dilution and dispersion step, the rotation speed of the stirring blade may be made faster than that in the kneading step. From the above, a negative electrode mixture can be prepared.

<< Second Step (S102) >>
In the second step, a negative electrode for a lithium ion secondary battery is manufactured using the negative electrode mixture. When the negative electrode mixture is made into a paint as described above, the negative electrode mixture is applied onto the negative electrode current collector by coating the paint on the negative electrode current collector and drying it. For coating, for example, a die coater, a gravure coater or the like can be used. The negative electrode current collector is, for example, a copper (Cu) foil. The negative electrode mixture after coating, that is, the negative electrode mixture layer may be compressed. For the compression, for example, a roll mill or the like can be used. Then, a negative electrode can be manufactured by cutting the whole into a predetermined dimension. For example, a slitter or the like can be used for cutting.

  The negative electrode current collector may be a foam metal base material made of, for example, Cu, nickel (Ni) or the like. In this case, the negative electrode can be produced by impregnating the foam metal base material with the paint. When the negative electrode mixture is a granulated body, for example, the granulated body can be formed into a sheet by compacting the granulated body between rolls, and the negative electrode can be produced.

  As described above, the negative electrode for a lithium ion secondary battery can be manufactured through the first step and the second step.

<Anode for lithium ion secondary battery>
FIG. 3 is a schematic diagram illustrating an example of the configuration of the negative electrode for a lithium ion secondary battery according to the present embodiment. The negative electrode 100 is a strip-shaped sheet member. The negative electrode 100 includes a negative electrode current collector 101 and a negative electrode mixture 102 disposed on the negative electrode current collector 101. The negative electrode mixture 102 is disposed on both main surfaces of the negative electrode current collector 101. The exposed portion 103 where the negative electrode current collector 101 is exposed from the negative electrode mixture 102 is provided for connection to an external terminal. On the negative electrode current collector 101, the negative electrode mixture 102 constitutes a negative electrode mixture layer. In the negative electrode 100, since the negative electrode mixture 102 contains at least a negative electrode active material and lithium tungstate, an improvement in Li precipitation resistance can be expected.

  As described in the method for producing a negative electrode for a lithium ion secondary battery, the negative electrode mixture contains lithium tungstate exceeding 0 μmol / g and not more than 201 μmol / g. The negative electrode mixture preferably contains lithium tungstate in an amount of 20 μmol / g to 200 μmol / g. More preferably, the negative electrode mixture contains 20 μmol / g or more and 135 μmol / g or less of lithium tungstate. The negative electrode mixture further preferably contains lithium tungstate in an amount of 20 μmol / g to 80 μmol / g. The negative electrode mixture most preferably contains lithium tungstate in a range of 32 μmol / g to 64 μmol / g. In the negative electrode composite material, the content of lithium tungstate can be measured using, for example, an ICP emission spectroscopic analyzer (ICP-AES) or the like.

  The negative electrode mixture preferably contains the above-described film forming agent (preferably at least one of VC and LiBOB). By allowing the negative electrode mixture to directly contain a film forming agent, a film derived from the film forming agent can be formed on the surface of the negative electrode active material during battery production. The effect of fixing lithium tungstate to the surface of the negative electrode active material can be expected by the coating. In addition, the coating can be expected to have an effect of suppressing elution of lithium tungstate. When the film forming agent is VC, the value obtained by dividing the content of VC by the content of lithium tungstate in the negative electrode mixture is preferably 0.85 or more and 1.2 or less. The equivalence is more preferably 0.90 or more and 1.2 or less.

<Method for producing lithium ion secondary battery>
Next, the manufacturing method of a lithium ion secondary battery is demonstrated. The said manufacturing method is a manufacturing method of a lithium ion secondary battery including the manufacturing method of the negative electrode for lithium ion secondary batteries mentioned above.

  That is, the method for producing a lithium ion secondary battery includes a first step (S101) for preparing a negative electrode mixture containing at least a negative electrode active material and lithium tungstate, and a negative electrode for a lithium ion secondary battery using the negative electrode mixture. The 2nd step (S102) which manufactures.

  FIG. 2 is a flowchart showing an outline of a method for manufacturing a lithium ion secondary battery. As shown in FIG. 2, the manufacturing method includes a negative electrode manufacturing step (S100), a positive electrode manufacturing step (S200), an electrode group manufacturing step (S300), a case housing step (S400), a liquid injection step (S500), and an initial charging step. A discharging step (S600) is provided.

<< Negative Electrode Manufacturing Step (S100) >>
In the negative electrode manufacturing step, a negative electrode is manufactured according to the above-described method for manufacturing a negative electrode for a lithium ion secondary battery. The same description is not repeated here.

<< Positive electrode manufacturing step (S200) >>
In the positive electrode manufacturing step, a positive electrode is manufactured. First, a positive electrode mixture containing a positive electrode active material, a conductive material, a binder, and the like is prepared. The positive electrode active material is not particularly limited. The positive electrode active material is, for example, LiCoO ₂ , LiNiO ₂ , a compound represented by the general formula LiNi _a Co _b O ₂ (wherein a + b = 1, 0 <a <1, 0 <b <1), _{LiMnO 2, LiMn 2 O 4,} ( a proviso Shikichu, a + b + c = 1,0 <a <1,0 <b <1,0 <c <1.) the general formula LiNi _a Co _b Mn _c O ₂ in Table Or a compound such as LiFePO ₄ . Examples of the compound represented by the general formula LiNi _a Co _b Mn _c O ₂ include LiNi _1/3 Co _1/3 Mn _1/3 O ₂ . The positive electrode mixture is prepared so as to contain, for example, about 80 to 98% by mass of the positive electrode active material.

  The conductive material may be carbon blacks such as acetylene black and thermal black. The positive electrode mixture is prepared so as to contain, for example, about 1 to 10% by mass of a conductive material. The binder may be, for example, polyvinylidene fluoride (PVdF), PTFE, or the like. The positive electrode mixture is prepared so as to contain, for example, about 1 to 10% by mass of the binder.

  Similar to the preparation of the negative electrode mixture described above, a mixing apparatus such as a planetary mixer can be used for the preparation of the positive electrode mixture. For example, N-methyl-2-pyrrolidone (NMP) can be used as the solvent.

  After preparing the positive electrode mixture, the positive electrode can be produced, for example, by coating the positive electrode mixture on the positive electrode current collector. When the positive electrode mixture is made into a paint, the paint may be applied on the positive electrode current collector using, for example, a die coater and dried. The positive electrode current collector is, for example, an aluminum (Al) foil. The positive electrode mixture after coating, that is, the positive electrode mixture layer may be compressed. For the compression, for example, a roll mill can be used. Then, a positive electrode can be manufactured by cutting the whole into a predetermined dimension. For example, a slitter or the like can be used for cutting.

  FIG. 4 is a schematic diagram illustrating an example of the configuration of the positive electrode. The positive electrode 200 is a strip-shaped sheet member. The positive electrode 200 includes a positive electrode current collector 201 and a positive electrode mixture 202 disposed on the positive electrode current collector 201. The positive electrode mixture 202 is disposed on both main surfaces of the positive electrode current collector 201. The exposed portion 203 where the positive electrode current collector 201 is exposed from the positive electrode mixture 202 is provided for connection to an external terminal. On the positive electrode current collector 201, the positive electrode mixture 202 constitutes a positive electrode mixture layer.

<< Electrode Group Manufacturing Step (S300) >>
In the electrode group manufacturing step, an electrode group is manufactured. FIG. 5 is a schematic diagram illustrating an example of the configuration of the electrode group. As shown in FIG. 5, the electrode group 800 is manufactured by laminating the negative electrode 100 and the positive electrode 200 with the separator 300 interposed therebetween, and further winding them. A winding device can be used to manufacture the electrode group. After winding, the electrode group may be formed into a flat shape using, for example, a flat plate press.

  Separator 300 may be a microporous film made of, for example, polyethylene (PE), polypropylene (PP), or the like. The separator may have a single layer structure or a multilayer structure. The separator may have a three-layer structure in which, for example, a PP microporous film and a PE microporous film are laminated in the order of PP / PE / PP. The separator may have a heat resistant layer containing an inorganic filler (for example, alumina or the like) on its surface.

<< Case Accommodation Step (S400) >>
In the case housing step, the electrode group is housed in the battery case. FIG. 6 is a schematic diagram illustrating an example of the configuration of a lithium ion secondary battery. The battery case 500 has a rectangular outer shape. The battery case is composed of, for example, a bottomed rectangular case body and a lid. The material of the battery case 500 may be, for example, an Al alloy. The battery case 500 includes external terminals 501 and 502. The battery case 500 may be provided with a liquid injection port, a safety valve, a current interruption mechanism, and the like. The electrode group 800 is housed in the battery case 500. When housed, the electrode group 800 is connected to the external terminals 501 and 502 at the exposed portions 103 and 203.

<< Liquid injection step (S500) >>
In the liquid injection step, an electrolytic solution is injected into the battery case. The electrolytic solution 600 can be injected from a liquid injection port provided in the battery case 500, for example. After the injection, the battery case is sealed with a predetermined means (for example, laser welding).

  The electrolytic solution is a liquid electrolyte in which a Li salt is dissolved as a supporting salt in an aprotic solvent. Examples of aprotic solvents include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and γ-butyrolactone (γBL), and dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC). And chain carbonates such as diethyl carbonate (DEC). Two or more of these aprotic solvents may be mixed to form a mixed solvent. In the mixed solvent, the volume ratio between the cyclic carbonates and the chain carbonates may be, for example, about cyclic carbonates: chain carbonates = 1: 9 to 5: 5. In such a range, the balance between electrical conductivity and electrochemical stability is good.

Examples of the Li salt include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), and lithium bis (trifluoromethanesulfonyl) imide. [Li (CF ₃ SO ₂ ) ₂ N: abbreviation “LiTFSI”], lithium bis (fluorosulfonyl) imide [Li (FSO ₂ ) ₂ N: abbreviation “LiFSI”], lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ) Etc. Two or more Li salts may be used in combination. The Li salt concentration in the electrolytic solution may be, for example, about 0.5 to 2.0 mol / l.

  The electrolytic solution may contain a film forming agent such as VC and LiBOB described above, and a gas generating agent such as cyclohexylbenzene (CHB) and biphenyl (BP).

<< Initial charge / discharge step (S600) >>
In the initial charge / discharge step, the first charge / discharge is performed. At this time, when the negative electrode mixture of the present embodiment contains a film forming agent, the film forming agent is reduced and decomposed, and a film derived from the film forming agent is formed on the surface of the negative electrode active material.

  In other words, the method for producing a lithium ion secondary battery includes a film derived from the film forming agent on the surface of the negative electrode active material by electrochemically reducing and decomposing the film forming agent contained in the negative electrode mixture. Can be included. As described above, the coating film can be expected to have an effect of suppressing elution of lithium tungstate.

  In this step, for example, a full range charge / discharge cycle with a low current value may be performed about 1 to 3 times. Charging and discharging may be performed using the CCCV method, for example. Specific charge / discharge cycle conditions are, for example, as follows.

Charging conditions: CC current value = 0.1C, CV voltage = 4.1V, cut current value = 0.01C
Discharge conditions: CC current value = 0.1 C, CV voltage = 3.0 V, cut current value = 0.01 C.

  Here, the unit “C” of the current value indicates a current value at which the rated capacity of the battery can be discharged in one hour. “CC” indicates a constant current, “CV” indicates a constant voltage, and “CCCV method” indicates a constant current-constant voltage method.

  Through the above steps, the lithium ion secondary battery 1000 can be manufactured.

<Lithium ion secondary battery>
In the lithium ion secondary battery manufactured by the above manufacturing method, the negative electrode includes a negative electrode mixture containing at least a negative electrode active material and lithium tungstate, and the negative electrode mixture exceeds lithium tungstate by 0 μmol / g. Contains up to 201 μmol / g. Therefore, the lithium ion secondary battery is a lithium ion secondary battery with improved Li deposition resistance. By improving the Li precipitation resistance, for example, the upper limit of the charging current can be set higher than before. This can be expected to improve the recovery efficiency of regenerative power or shorten the charging time.

  In the lithium ion secondary battery, the negative electrode mixture constituting the negative electrode preferably contains a film derived from the film forming agent. The coating is preferably derived from at least one of VC and LiBOB. The coating is preferably attached to the surface of the negative electrode active material. It is preferable that the coating also adheres to lithium tungstate. That is, the negative electrode mixture preferably contains a negative electrode active material and a film attached to lithium tungstate. The coating can be expected to have an effect of fixing lithium tungstate to the surface of the negative electrode active material. In addition, the coating can be expected to have an effect of suppressing elution of lithium tungstate, for example, during high temperature storage.

  Based on the above characteristics, the lithium ion secondary battery is considered suitable for in-vehicle use. The rated capacity of the lithium ion secondary battery is, for example, about 1 to 30 Ah (typically about 4 to 25 Ah).

  In the above, this embodiment was demonstrated taking the square battery as an example. However, the above description is merely an example, and the present embodiment is not limited to the prismatic battery. This embodiment may be applied to, for example, a cylindrical battery, a laminated battery, and the like. The electrode group is not limited to a wound type. The electrode group may be a stacked type (also referred to as “stacked type”).

  Hereinafter, although this embodiment is described using an example, this embodiment is not limited to these examples.

<Experiment 1: Examination of Lithium Tungstate Content in Negative Electrode Mixture>
In Experiment 1, the content of lithium tungstate was examined. Specifically, No. 1 is as follows. 1-No. The negative electrode concerning 21 and the battery (rated capacity 5Ah) using this were manufactured, and Li precipitation tolerance was evaluated.

<< No. 1 >>
1. Negative electrode manufacturing step (S100)
1-1. First step (S101)
First, the following materials were prepared: Negative electrode active material: amorphous coated spheroidized natural graphite
(BET specific surface area = 2.9 m ² / g, average particle size = 10 μm)
Lithium tungstate: Li ₂ WO ₄ (average particle size = 3 μm)
Thickener: CMC-Na
Binder: SBR
Solvent: Water Negative electrode current collector: Cu foil Here, “amorphous coated spheroidized natural graphite” refers to a composite in which spheroidized natural graphite particles are further coated with amorphous carbon Shows particles.

1-1-1. Solidification step Each material was added to the mixing vessel of the planetary mixer with the following composition. Thickener: 1% by mass (value relative to the final negative electrode mixture)
Lithium tungstate: 20 μmol / g (value for the final negative electrode composite)
Negative electrode active material: remainder (value with respect to final negative electrode mixture).

  A solvent was added to the mixing container and the mixture was kneaded so that the solid content ratio of the mixture was 65% by mass.

1-1-2. Dilution and dispersion step Next, a solvent was added and further mixed so that the solid content ratio of the mixture was 50% by mass. At this time, 1% by mass (value relative to the final negative electrode mixture) was also added to the mixture.

  From the above, a negative electrode mixture containing at least a negative electrode active material and lithium tungstate was prepared. The negative electrode mixture is made into a paint.

1-2. Second step (S102)
Using the die coater, the paint obtained above was applied onto both main surfaces of the negative electrode current collector and dried. The negative electrode mixture after drying was compressed using a roll mill. Furthermore, it cut | disconnected to the predetermined dimension using the slitter. From the above, the negative electrode 100 shown in FIG. 3 was manufactured.

2. Positive electrode manufacturing step (S200)
The positive electrode 200 shown in FIG. 4 was manufactured using the following materials. The composition of the positive electrode mixture was positive electrode active material: conductive material: binder = 90: 8: 2 by mass ratio.

Cathode active material: LiNi _1/3 Co _1/3 Mn _1/3 O ₂
Conductive material: Acetylene black Binder: PVdF
Solvent: NMP
Positive electrode current collector: Al foil.

3. Electrode group manufacturing step (S300)
A separator having a three-layer structure of PP / PE / PP was prepared. Using a winding device, a negative electrode and a positive electrode were laminated with a separator interposed therebetween, and these were further wound. After winding, the electrode group was formed into a flat shape using a flat plate press. Thereby, the electrode group 800 shown in FIG. 5 was manufactured.

4). Case accommodation step (S400)
A rectangular battery case 500 shown in FIG. 6 was prepared. The electrode group 800 was connected to the external terminals 501 and 502, and the electrode group 800 was accommodated in the battery case 500.

5). Injection step (S500)
An electrolytic solution 600 having the following composition was injected into the battery case 500. After injection, the battery case 500 was sealed Li salt: LiPF ₆ (1.1 mol / l)
Solvent: [EC: DMC: EMC = 3: 4: 3 (volume ratio)].

6). Initial charge / discharge step (S600)
Three charge / discharge cycles under the following conditions were executed. In this experiment, the discharge capacity at the third cycle is the initial capacity. Charging conditions: CC current value = 0.1 C, CV voltage = 4.1 V, cut current value = 0.01 C
Discharge conditions: CC current value = 0.1 C, CV voltage = 3.0 V, cut current value = 0.01 C.

From the above, no. A negative electrode according to 1 and a battery using the same were manufactured.
<< No. 2-No. 21 >>
As shown in Table 1, except that the BET specific surface area of the negative electrode active material and the content of lithium tungstate were changed, No. 1 was obtained. In the same manner as in No. 1, 2-No. A negative electrode according to 21 and a battery using the same were manufactured.

  In Table 1, no lithium tungstate is contained or the content of lithium tungstate exceeds 201 μmol / g. 7, no. 13, no. 14, no. 20, no. 21 is a comparative example, and the others are examples.

<Evaluation of battery performance>
Using the limit current value of the battery as an index, the Li deposition resistance of the battery was evaluated. The procedure for measuring the limit current value is as follows.

  First, 100 pulse charge / discharge cycles in which the combination of pulse charge for 20 seconds at the first current value and pulse discharge for 20 seconds are taken as one cycle are performed. After 100 cycles, the post-cycle capacity is measured in the same manner as the initial capacity. The capacity retention rate (percentage) is calculated by dividing the post-cycle capacity by the initial capacity. If the capacity maintenance ratio is 90% or more, the current value is changed to a second current value larger than the first current value (for example, about 1.1 times the first current value), and the pulse is changed in the same manner as described above. The charge / discharge cycle and the capacity maintenance rate are calculated.

  By repeating these operations, a limit current value at which the capacity maintenance ratio can be maintained at 90%, that is, a limit current value is obtained. It is considered that the decrease in capacity retention rate accompanying the pulse charge / discharge cycle is due to Li precipitation. Therefore, a large limit current value indicates that Li precipitation hardly occurs even with a large charging current, that is, Li precipitation resistance is good.

The measurement results are shown in Table 1 and FIG. In Table 1, No. 1-No. The numerical value shown in the column of “limit current value ratio” in No. 7 is No. in which the negative electrode mixture does not contain Li ₂ WO ₄ . The limit current value of 7 is 1, and the relative value (dimensionless number) is shown.

No. 8-No. No. 14 in the “Ratio of limit current value” column is No. in which the negative electrode mixture does not contain Li ₂ WO ₄ . The limit current value of 14 is 1, and the relative value is shown.

No. 15-No. The numerical values shown in the column “Ratio of limit current values” in No. 21 are No. in which the negative electrode mixture does not contain Li ₂ WO ₄ . The limit current value of 21 is 1, and the relative value is shown.

"Results and discussion"
FIG. 7 is a graph showing the relationship between the content of lithium tungstate in the negative electrode mixture and the limit current value. The vertical axis of FIG. 7, "No.1~No.7", in each sample group "No.8~No.14" and "No.15~No.21", containing no Li ₂ WO ₄ Sample The limiting current value of 1 is 1, and the relative value is shown. The horizontal axis of FIG. 7 shows the content of Li ₂ WO ₄ in the negative electrode mixture.

From Table 1 and FIG. 7, it can be seen that the limit current value is increased by adding Li ₂ WO ₄ to the negative electrode mixture. This is probably because the presence of lithium tungstate increases Li ions around the negative electrode active material and improves the reactivity of the negative electrode active material. However, when the content of Li ₂ WO ₄ exceeds 201 μmol / g, the limit current value is rather lowered. Therefore, the Li ₂ WO ₄ content needs to be 201 μmol / g or less.

In this experiment, it was confirmed that the effect was large in the range where the content of Li ₂ WO ₄ was 20 μmol / g or more. Therefore, it is considered that the content of Li ₂ WO _{4 in} the negative electrode mixture is preferably 20 μmol / g or more.

From Table 1 and FIG. 7, the improvement width of the limiting current value tends to be larger when the content of Li ₂ WO _{4 in} the negative electrode mixture is 20 μmol / g or more and 135 μmol / g or less. Further, when the content of Li ₂ WO _{4 in} the negative electrode composite is 20 μmol / g or more and 80 μmol / g or less, the improvement range of the limit current value is further increased. From Table 1 and FIG. 7, the range where the greatest effect can be expected is considered to be a range of 32 μmol / g or more and 64 μmol / g or less.

<Experiment 2: Examination of content of film forming agent in negative electrode mixture>
In Experiment 2, the content of the film forming agent was examined. Specifically, No. 1 is as follows. 22-No. A negative electrode according to No. 28 and a battery using the same were manufactured, and the Li deposition resistance after storage was evaluated.

<< No. 22-No. 28 >>
No. 22 to 28, a negative electrode active material having a BET specific surface area of 4.1 m ² / g was used, and the content of lithium tungstate in the negative electrode mixture was 43 μmol / g.

Vinylene carbonate (VC) was prepared as a film forming agent. VC was added to the mixture along with the negative electrode active material and the like in the kneading step. As shown in Table 2, various negative electrode composites were prepared by changing the amount of VC in each sample. In Table 2, the numerical value shown in the column of “(VC content) ÷ (Li ₂ WO ₄ content)” is the content of vinylene carbonate in the negative electrode mixture, and the content of lithium tungstate in the negative electrode mixture. The value (dimensionless number) divided by. Except these, the above-mentioned No. In the same manner as in No. 1, 22-No. A negative electrode according to No. 28 and a battery using the same were manufactured. No. 22-No. 28 is an example.

<Evaluation of battery performance>
In the same manner as in Experiment 1, the initial limit current value was measured. The results are shown in Table 2. In Table 2, “Limit current value ratio” shown in the “Initial” column is “No. The initial limit current value of 25 is 1, and the relative value (dimensionless number) is shown.

  After measuring the initial limit current value, in Experiment 2, the limit current value after high-temperature storage was measured as follows. The SOC (state of charge) of the battery was adjusted to 80%. The battery was placed in a thermostat set to 60 ° C. In this state, the battery was stored for 60 days. After 60 days, the limit current value was measured in the same manner as in the initial stage. The results are shown in Table 2 and FIG. The “ratio of limit current values” shown in the column “After high temperature storage” in Table 2 is “No. The initial limit current value of 25 is 1, and the relative value is shown.

"Results and discussion"
FIG. 8 is a graph showing a relationship between a value obtained by dividing the content of VC in the negative electrode mixture by the content of lithium tungstate in the negative electrode mixture, and a limit current value after high-temperature storage. The vertical axis in FIG. The initial limit current value of 25 is 1, and the relative value is shown. The horizontal axis of FIG. 8 shows a value obtained by dividing the content of VC in the negative electrode mixture by the content of lithium tungstate in the negative electrode mixture.

  From Table 2 and FIG. 8, the negative electrode mixture was adjusted so that the value obtained by dividing the content of VC in the negative electrode mixture by the content of lithium tungstate in the negative electrode mixture was 0.85 or more and 1.2 or less. It turns out that the fall of Li precipitation tolerance after high temperature storage can be suppressed significantly by preparing. It is thought that the elution of lithium tungstate is suppressed by the coating derived from VC.

  From Table 2 and FIG. 8, it is considered that the value obtained by dividing the content of VC in the negative electrode mixture by the content of lithium tungstate in the negative electrode mixture is more preferably in the range of 0.90 to 1.2.

  In this experiment, VC was used as the film forming agent, but the same effect can be expected even when LiBOB is used.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 100 Negative electrode (negative electrode for lithium ion secondary batteries), 101 Negative electrode collector, 102 Negative electrode compound material, 103,203 Exposed part, 200 Positive electrode, 201 Positive electrode current collector, 202 Positive electrode compound material, 300 Separator, 500 Battery case, 501,502 External terminal, 600 electrolytic solution, 800 electrode group, 1000 lithium ion secondary battery.

Claims (8)

A first step of preparing a negative electrode mixture containing at least a negative electrode active material and lithium tungstate;
A second step of producing a negative electrode for a lithium ion secondary battery using the negative electrode mixture,
The manufacturing method of the negative electrode for lithium ion secondary batteries which prepares the said negative electrode compound so that the said negative electrode compound may contain the said lithium tungstate more than 0 micromol / g and 201 micromol / g or less in the said 1st step.   The method for producing a negative electrode for a lithium ion secondary battery according to claim 1, wherein in the first step, the negative electrode mixture is prepared so that the negative electrode mixture contains 20 µmol / g or more of the lithium tungstate.   In the first step, the negative electrode mixture is prepared so that the negative electrode mixture further contains a film forming agent that forms a film on the surface of the negative electrode active material by an electrochemical reduction reaction. The manufacturing method of the negative electrode for lithium ion secondary batteries of Claim 1 or Claim 2.   The method for producing a negative electrode for a lithium ion secondary battery according to claim 3, wherein the film forming agent is vinylene carbonate.   In the first step, a value obtained by dividing the content of the vinylene carbonate in the negative electrode mixture by the content of the lithium tungstate in the negative electrode mixture is 0.85 or more and 1.2 or less. The manufacturing method of the negative electrode for lithium ion secondary batteries of Claim 4 which prepares the said negative electrode compound material.   The method for producing a negative electrode for a lithium ion secondary battery according to claim 3, wherein the film forming agent is lithium bis (oxalate) borate. A negative electrode mixture containing at least a negative electrode active material and lithium tungstate,
The negative electrode composite material is a negative electrode for a lithium ion secondary battery, containing the lithium tungstate in an amount of more than 0 μmol / g and not more than 201 μmol / g.   The negative electrode for a lithium ion secondary battery according to claim 7, wherein the negative electrode mixture contains 20 μmol / g or more of the lithium tungstate.